It is possible to control combustion processes in diesel engines to limit the number of cylinders providing torque output. One technique is cylinder cut-out. The technique eliminates fuel to a cylinder while continuing to cycle the intake and exhaust valves. The piston also cycles. The technique results in fuel economy losses.
At very low loads and idle conditions, an engine runs with poor fuel efficiency. All cylinders are firing, but little to no torque output is needed. Even in a loaded idle condition, the engine could provide more torque than necessary. Fuel is wasted, and the fuel economy is poor.
The low, and inefficient, fuel use is not effective to heat the aftertreatment system, and so pollution is high.
It would be beneficial to improve fuel economy and fuel efficiency in a diesel engine. It is beneficial to reduce pollution.